1. Field of the Invention
The present invention relates generally to a method of die cast molding a metal directly onto a fiber reinforced plastic body to form a structure. In particular, a structure formed pursuant to this method includes a preselected failure site to control separation of the cast metal from the plastic body when the structure is subjected to excessive tensile loads.
2. Description of the Related Art
Links, generally formed as elongated metallic members having eyelets on each end, are well-known in the automotive industry. In particular, links are used to connect various components in a suspension system. In use, a link can be subject to compressive, tensile and shear loads.
It is desirable to substitute lighter materials for traditional metals such as aluminum to form links. Fiber reinforced plastic, typically referred to as FRP, may find increasing usage in the automotive industry, despite its higher cost, because of its high strength to weight ratios. However, one problem with substituting FRP for metal in any automotive component is the fact that it is difficult or impossible to form it into shapes that are convoluted or discontinuous. Thus, it may serve well as a drive shaft, which is an elongated tube of constant cross section, but not as a transmission case, with its labyrinthine internal passages.
Another limitation is that many automotive components must be attached directly to another metal component at some point, which may require that the FRP component be provided with a localized metal fastening member. For example, an FRP drive shaft must have a metal connector at each end for attachment to the remainder of the drive line. It is difficult to successfully and securely mate FRP directly to metal, especially when the attachment point will be subject to heavy loading and stress. Many patents are directed just to the problem of joining metal end pieces to FRP drive shafts, most of which involve various adhesives, rivets, splines or combinations thereof.
The designer of an FRP link would face both problems noted above. The main body of a link is basically a rod or beam with a fairly constant cross section and smooth exterior surface, presenting no particular protrusions or discontinuities. This is a basic shape that would lend itself well to FRP manufacture. A matrix of full length reinforcing glass fibers soaked with a conventional thermosetting resin is formed in a mold with the desired beam shape, and then heat cured. However, each end of the beam must be connected to other structures, e.g., between a suspension support and a wheel assembly support. Die casting a metal eyelet directly to the end of an FRP beam would be preferable, in terms of time, cost and strength, to attaching a separate connector by adhesive or mechanical means. However, the thermoset resin that binds the fibers together decomposes badly at the melting temperatures of suitable metals, such as aluminum alloy. Tests that subjected FRP to molten metal for times comparable to the cycle times involved in standard die casting operations found such severe thermal decomposition of the resin as to conclude that the process would not be feasible.
A particular aspect of a joint between an FRP body and a metal must be addressed when the component is subject to tensile loads. Under excessive tensile loads, the metal may completely pull away from the FRP member. If the component is a link, e.g., a FRP rod connected to a metal eyelet, complete separation of the eyelet from the rod under excessive tensile loads is unsatisfactory.